Plutonium-cerium alloy



Jan. 6, 1959 A. s. OFFINBERRY PLUToNIUM-CERIUM ALLOY 2 sheet'ssheet 1 Filed Aug. 29, 1957 2D-mmc /.momma O EO. di @o om om o om om Q om om o i l M. Illu OON x w+ f oom mola :nT :nT 1 oow 0| u an. w 1 oom Y om Mwons D503 oo y IN VEN TOR. ARTHUR S. CUFFIA/BERRY W/ TNESSESJ www Filed Aug. 29, 1957 A. s. coFr-INBERRY PLUTONIUM-CERIUM ALLOY l l x I l I WTNESSES= IN V EN TOR.

ARN-IUI? S. CFT'/VBERRY PLUTONIUM-CERIUM ALLOY Arthur S. Cotiinberry, Los Alamos, N. Mex., assignor to the United States of America as represented by the United States Atomic Energy Commission Application August 29, 1957, serial No. 681,149

2 claims. (c1. 1s-122.7)

The present invention relates to alloys of plutonium and more specifically to alloys of plutonium which are useful in neutronic reactors as the fuel of such reactors.

Neutronic reactors `havel been constructed and operated in which the fuel element has been essentially pure plutonium. F or example, such a reactor is the Los Alamos fast reactor known as Clementine Such reactors use a fairly small volume of fuel because of the great activity of pure plutonium and consequently the heat developed is also confined to a very small volume with the attendant ditiiculties of removing a large quantity of heat from a small volume. (This reactoi is described in AEC LA- 1679 available from TIS, Oak Ridge, Tenn.)

In a reactor using uranium for the fuel elements, where the fuel is actually atoms of uranium of the isotope U235, an expansion of the fuel volume is achieved by controlling the degree of enrichment of U235 within a matrix of U238 and this enrichment is achieved by limiting the degree of isotope separation of U235. Since plutonium is produced as a pure material almost exclusive of any isotope other than Pu239, this same automatic dilution is not achieved. Thus to gain the advantages of dispersed fuel it is necessary to add to the plutonium atoms some foreign element. The choice of a foreign element is not a matter which can be settled by the selection of almost any available metal because of the following strict requirements which are the least that must be met for reactor use. These are:

(1) A neutronic compatability with the system. The

The preferred embodiment'of the present invention comprises an alloy system of plutonium and cerium consisting of from 2 to 90 atomic percent cerium and the balance plutonium.

Preparation The alloys of the present invention are, in the preferred embodiment, prepared and cast in a vacuum. The constituents can be placed in a tantalum crucible in a vacuum furnace wherein a vacuum of the order of 10*4 mm. of Hg can be maintained throughout the melting and casting operations. Crucibles of magnesium oxide as produced by the method of co-pending application S. N. 597,829, tiled July 13, 1956, Allison, can beused, for example. Before use, Vthe Crucibles are degassed at 1100 to 1200 C. to assure a high purity of the reactor fuel.

A suitable melting cycle consists of heating the alloying constituents above the melting point (as seen in Fig. 1) quite rapidly with an induction furnace of sufiicient power to do so. The molten mixture is held at this temperature for about l5 minutes and then allowed to cool at the natural rate of the furnace (approximately 5/min.). This procedure was found to be satisfactory for the production of all alloys claimed in the present invention.

If it is desired to cast alloys in a form other than the natural shape of the crucible, they can be cast when molten or may be cooled in the Crucible, remelted and cast. Best homogeneity of the alloy is obtained by chill casting, i. e., very rapid freezing by casting into a water element must not be a strong absorber of neutrons in the energy range of the neutrons of the reactor system.

(2) It must improve the fabrication characteristics of plutonium. The element must produce an alloy which can be fabricated to fuel elements with considerable ease.

(3) It must alloy with plutonium readily. The element must alloy with plutonium readily so that the preparation of such an alloy is a simple metallurigical step.

(4) It must have reasonable radiation stability. As is well known, any fuel element subjected to a high flux of neutron and gamma rays will change its characteristics due to nuclear changes within the crystal structure of the element. Some materials exhibit a greater resistance to changes under these conditions than others.

It has been found that the metal cerium when alloyed with plutonium in a wide range of percentages will produce an alloy which meets the above stringent requirements.

It is therefore an object of this invention to provide an alloy of plutonium and cerium having good neutronic and fabrication characteristics.

It is a further object of this invention to provide an alloy of plutonium and cerium having good radiation resistant characteristics.

Further objects of this invention will be apparent from the following description and claims and Figures 1 and 2 hereby made a part of the specification in which Figure l shows a phase diagram of the plutonium-cerium system, and Figure 2 shows a computed curve of the increase in the amount of Pu needed for a critical mass of alloy.

cooled copper mold. Castability of these alloys is excellent. The techniques used in casting pure aluminum are usually applicable to these alloys.

It has been found that an alloy containing about 15 percent or less cerium will, as cooled, contain its plutonium in the delta phase (see Fig. 1). The delta phase plutonium in this region is metastable and as the alloy undergoes considerable shock which might be produced by machining, for example, the plutonium will change to the alpha phase. This does not in any way affect the machining qualities of the alloy except possibly to improve them.

Properties 0f the alloys As can be seen from the phase diagram of Figure 1 an alloy composed of from slightly above zero percent cerium to about 15 percent cerium consists of the delta phase plutonium in a metastable condition. From about 15 percent to about percent cerium the alloy consists of a mixture of delta plutonium and alpha phase cerium.

The alloy in either the delta phase plutonium or the delta plutonium plus alpha cerium has good qualities for ease of fabrication for reactor fuel purposes. Its consistency is similar to lead and consequently the alloy may be extruded or pressed into desired shapes with considerable ease.

A great problem in reactor design is the selectionv of materials which can undergo high neutron flux bombardment without premature deleterious effects. A material which is relatively soft initially can withstand a large amount of neutron bombardment without becoming unusably brittle very quickly, as the material has a long range of hardness change to undergo before it becomes too brittle. The alloy of the present invention has this desirable characteristic since it is quite soft initially.

The nuclear properties of cerium are sufliciently excellent so that alloys containing up to 90 percent cerium may be used as a reactor fuel. Elements which absorb many neutrons in any manner would not be suitable for reactor fuel in this large a percentage. When plutonium is alloyed with another element, there is an increase in the amount of plutonium needed for a critical mass of alloy. This increase depends on the volume occupied by the alloy and the neutronic characteristics of the alloying it can be seen that a perfect but unobtaira'bl'eitioiwouldl be unity for all alloys. 'Thus it `is"important to keep R as close to unity as possible. Cerium is. an excellent element for keeping Rplow, as can be seen in lFigure 2. Most elements have an R'considerably above cerium for any given percentage.

' Gene'ral considerations When' an-alloy 'of the present invention containsa cnsiderableamount f' ceriurn 'thefmass of alloy require-d in a given' neutronic vreactorfor the's'a'rne reactivity conditions wilLlofcourse-bea great deal higher than `pure plutonium. This,asv has Vbeens'tat'edj{52'1'1'1 be ya vdesired advantage -in `-rea`c`tr design primarily vbecause the'fvolume ofV material Avin `Whichfthev heat of the reactor -is born 'is considerably greater'- and -theease'v of removing such Iheat is'euu'ally easier. F urthe'rmo're; thermal conductivity-'and fabricability ofthealloy `'are better than pure'plutonium. If the pltonium-'cerium -alloyisfu'sedfin a reactor such as Clementine I'the vnumber of fuell rods-will be 'increased'in'an amount'which can be determined by methods v/ell-k'no'wn'inf'thel art. In-th`atparticular designnthe added fuel rods can be placed in a volume'g-ained-by using fewer 'uranium rods withinrthetfuel cage. In other reactors currently being designed elsewhere, vto which reference "canibe 'found in' `tlielitera'ture,` the v"fuel cavity can usually be`de`signed to accmmodatethe fuel volume for the plutonium-cerium valloysy Without any radical change in design.

ltshould'be noted `that existing neutronic reactors do not operate at temperatures above 550 C. and that the alloy of the present invention when used as a solid fuel material should not be operated through a temperature above 550 C. if the alloy contains from about 15 atomic percent cerium to about-8O` atomic percent cerium. The reason for this is that at 550 C. in this percent cerium range, the delta phase transformsto the epsilon phase with a sudden change in volume. vIn a reactor, a sudden change in volume is undesirable.

As can be seen in the phase diagram of Figure l, the

specification discloses alloys of plutonium and cerium in the range of zero to 100"perce`nt cerium. 'Howeven the beneficial effects of ceriumjbecorne noticeable at about 5 percent cerium and althoughthe fabrication advantages exist up to 100 percent cerium, the nuclear considerations require that a fuel forca nuclear reactor have not more than 90'percntl-ceriumfwith the balance plutonium.

Therefore'"the-present invention is not limited by the foregoing lspe'cication,`but only-by theffollowing claims whichV 'recog'riiie u the variations Within -the* spirit of this invention, :which may be -developed byexercising vskill inthea'rt; A

What-is claimed is:

nl. An alloy for a neutr'onic reactor fuel consisting essentially of from'S -to 90 atomic percent cerium and the balance plutonium.

2."'An=alloy foraneutrcnr'iic'v reactor fuel for operation below 550 C. consisting ess'e'ritiallyof from about l5 to about 80 atoni'ic'perc'ent'cerium and thebalance plut'o'niun.

`No`references cited. 

1. AN ALLOY FOR A NEUTRONIC REACTOR FUEL CONSISTING ESSENTIALLY OF FROM 5 TO 90 ATOMIC PERCENT CERIUM AND THE BALANCE PLUTONIUM. 